Plans by a London-based mining company for a massive gold and copper mine in Alaska are "too risky" for the environment, local communities have warned.

Native American and community representatives are in London for Anglo-American's AGM this week to oppose proposals for a open pit mine in the Bristol Bay watershed which they say threatens the world's most valuable wild sockeye salmon fishery.

In the latest twist in the debate over the plans, the US Environmental Protection Agency (EPA) has said it will complete a scientific assessment into how future large-scale development projects may affect water quality and the salmon fishery.

Kimberley Williams, a subsistence fisherwoman and executive director of Nunamta Aulukestai, an association of Bristol Bay native village corporations, said the local community was calling on the EPA to protect the salmon and people of the bay.

Ms Williams said she was in London to warn shareholders and the board against investing in the "risky" project that may never go ahead.

She said the fishery was 100 years old, and had been relied on by indigenous people for subsistence fishing for even longer, and any contamination from the Pebble Mine project would hit the river systems.

"We have so much to lose, it's too risky, it's unacceptable to us," she warned.

Nunamta Aulukestai spokesman Bobby Andrew said the site - which would be North America's largest open pit mine - would be in a "very eco-sensitive", as well as seismically-active, region.

"It's in the wrong place," he said.

"It's going to have an impact on the fisheries and all the other renewable resources in the area. The risk is very high of it damaging the fisheries, and it's not just the fisheries, it's the tourism industry that relies on the salmon as well."